The present invention relates generally to semiconductor wafer processing, and more particularly to wafer carriers for retaining semiconductor wafers during processing operations.
In conventional wafer processes for removing wafer material, such as a double-side polishing operation, a wafer carrier is used to retain a plurality of wafers during the polishing operation. The wafer carrier is typically a thin, flat plate disposed between polishing pads of the polishing machine. The plate has teeth on its outer edge for engaging outer and inner pin ring drives adapted to rotate the plate during polishing. The wafer carrier is typically made of metal in order to withstand the mechanical stresses caused by the ring drives. However, during the latter stages of polishing, the pads polish not only the wafers, but also the carrier, and thereby release metal ions from the carrier. Such metal ions then enter the slurry and polishing pads and can cause bulk metal contamination of the wafers. Metals of particular concern are copper and nickel.
Plastic or fiber-reinforced plastic carriers are superior to metal carriers in terms of bulk metal contamination of the wafers, but the reduced strength of such carriers makes them unreliable. Plastic-coated metal carriers are generally unreliable because the plastic tends to delaminate, thus exposing the metal and scratching the wafers. It has been suggested to reduce bulk metal contamination by attempting to ensure that polishing of the wafer is stopped before the wafer thickness is the same as that of any metal portion of the carrier. However, such polishing requires the use of lower pad pressure against the wafers (which reduces polishing efficiency) to avoid rounding at the edges of the wafer. Therefore, such polishing is not ideal for efficient throughput or for producing the flattest wafers possible.